The present invention relates to a process for producing a high-monoester sucrose higher fatty acid ester (a higher fatty acid ester of sucrose with a high monoester content) which is of value as a solubilizer.
It is widely known that sucrose higher fatty acid esters are good food emulsifiers which are very safe and offer a broad range of HLB numbers ranging from high hydrophilicity to high lipophilicity according to the degree of esterification.
Particularly, sucrose higher fatty acid esters rich in monoester and diester components and, thus, having high HLB numbers are very effective emulsifiers for the formation of oil-in-water emulsions and it is also known that the monoesters have high solubilizing powers. However, the monoester content of the sucrose fatty acid esters commercially available today is generally not more than about 80%.
This is because of the following circumstances. The production technology heretofore used commercially for the production of a sucrose fatty acid ester involves a transesterification reaction between a fatty acid lower alcohol ester and sucrose (e.g. U.S. Pat. No. 2, 893,990), an acylation reaction between sucrose and a fatty acid halide or anhydride or an esterification reaction in the presence of an enzyme such as lipase. However, since 3 of the 8 hydroxyl groups of sucrose are primary and very reactive, it is difficult to selectively synthesize a monoester. Moreover, even if the starting material sucrose is used in large excess over the fatty acid, it is impossible to prevent production of the diester and triester.
Therefore, several procedures have been proposed for separating a sucrose higher fatty acid monoester from a mixture including the diester and triester.
By way of example, column chromatography or thin-layer chromatography using a halogenated hydrocarbon or methanol as the solvent and an extraction procedure using dimethylformamide, n-hexane, a halogenated hydrocarbon, methanol, water or the like as the solvent (e.g. Japanese Patent Publication No. 40-24606) are known.
However, the chromatographic method is disadvantageous in that it not only requires a large quantity of the solvent but is restricted in the batch size that can be treated. Regarding the extraction method, too, the use of n-hexane, a halogenated hydrocarbon or methanol presents a safety problem and particularly when methanol is used as the solvent, decomposition of the sucrose higher fatty acid ester by methanolysis is said to be unavoidable.